This invention relates to the field of digital image processing, and more particularly to methods and apparatus for producing very low resolution, low bit depth images.
It would be desirable to display consumer images on the display panels of cellular telephones and personal digital assistants (PDA""s). Unfortunately, the displays on currently available cellular telephones and personal digital assistants have a very low resolution (e.g. 95xc3x9745 to 302xc3x9798 pixels) and low bit depth (e.g. 1 bit). A digital image of a high resolution (e.g. 1536xc3x971024 pixels) and a high bit depth (e.g. 24 bits) can be produced by a digital camera or by scanning a consumer film. If it is subsampled to the resolution of the small display and halftoned to produce a 1-bit depth binary image, the features in the resulting image are rendered indistinctly.
One approach to producing a low resolution, low bit depth digital image is to first reduce the bit depth of the image using the xe2x80x9cstamp filterxe2x80x9d available in Adobe Photoshop(trademark), and then sub-sample the image to reduce the resolution. It is believed that the stamp filter employs a rank order filter and a thresholding operation to reduce the bit depth of the image. The resulting images, when displayed on a cellular telephone or PDA, tend to have too much dark areas, obscuring details in these dark areas.
There is a need therefore for an improved method of processing digital images to produce low resolution, low bit depth images.
The need is met according to the present invention by providing a digital image processing method for generating a low resolution, low bit depth digital image from a higher resolution, higher bit depth input color digital image, that includes the steps of: generating a gray scale image from the input color digital image; generating a valley image; filtering the valley image with a rank order filter to produce a filtered valley image; thresholding the filtered valley image to generate a binary valley image; generating a shadow image and a highlight image from the gray scale image; combining the shadow image, the highlight image, and the binary valley image to produce a combined binary image; and sub-sampling the combined binary image to a desired low resolution.
The present invention has the advantage of producing a low resolution low bit depth digital image that is recognizable for its scene/event content on a display device with limited display area and bit depth, and enables efficient compression and transmission of images to mobile devices having low resolution displays.